The present invention relates to processes for winning aluminum metal from aluminous ore. It particularly concerns nonelectrolytic methods for recovering aluminum, thereby reducing the levels of energy consumption.
The Bayer-Hall process has been used for the recovery of aluminum from ore for well over half a century. It is at present the only process commercially used in the United States to produce aluminum from bauxite. In the first step of the Bayer process, finely ground bauxite is dissolved in a mixture of sodium aluminate (NaAlO.sub.2) and free caustic at elevated temperature and pressure. The other materials present in the bauxite, namely hematite (Fe.sub.2 O.sub.3), titania (TiO.sub.2) and silica (SiO.sub.2) are insoluble. These impurities are removed from the aluminate liquor by thickening and filtration and then discarded.
The aluminum-bearing liquor is diluted and then cooled before being sent to large precipitation tanks where alumina (as trihydrate) precipitates out of solution. The solids are washed and then calcined to produce the alumina feed for the aluminum smelter. The spent liquor is concentrated by evaporation and recycled to the bauxite dissolution.
Reduction of alumina to aluminum is accomplished by the Hall electrolytic process. Alumina is continuously dissolved in a molten cryolite (typically a mixture of fluorides of sodium, aluminum and calcium) in an electrolytic cell where aluminum is produced at the cathode and oxygen-containing gases at the anode. The aluminum product, which collects on the bottom of the cells, is periodically removed and cast into ingots.
The Bayer-Hall process and various other electrolytic processes require relatively high expenditures of energy. At present, the aluminum industry uses about 4% of the nation's total electrical energy, thus establishing considerable incentive for more energy efficient means of producing aluminum.
One other process under active consideration by the aluminum industry involves the conversion of alumina to a material such as aluminum chloride (AlCl.sub.3) followed by the electrolysis. This chloride process starts with alumina from the Bayer process. The alumina is first impregnated with carbon from cracking of heavy fuel oil in a two-stage fluid bed. The carbon-impregnated alumina is then chlorinated to produce volatile aluminum chloride in a fluid bed reactor at about 1300.degree. F. The fluid-bed off-gases are cooled to condense out impurities. A final condensation, at about 150.degree. F., removes aluminum chloride. The aluminum chloride is then fed to electrolytic cells containing a mixture of chlorides of aluminum, sodium and lithium operating at about 1290.degree. F. Inert carbon anodes and cathodes are used to produce aluminum at the cathode and chlorine at the anode. The chlorine is recycled to the chlorination step. The molten aluminum is withdrawn from the cells and cast into ingots.
Even though the electrolytic decomposition of aluminum chloride requires somewhat less energy expenditure than the traditional alumina electrolysis, the preliminary process steps of purification and chlorination of alumina offset these economies to a large extent.